This study will determine the safety of transdermal estrogen therapy in post-menopausal women previously treated for porphyria cutanea tarda (PCT), determine the effects of transdermal and oral estrogen therapy on porphyrin metabolism in normal, post-menopausal women and of transdermal estrogens in post-menopausal women previously treated for PCT and determine the activity of hepatic P4501A2 activity in women with PCT and controls.